The present invention relates to a regulating system for the regulation of the driving torque of the internal combustion engine of a motor vehicle, in which wheel slippage occurring at the driven wheels of the motor vehicle is detected according to magnitude and in which upon exceeding a slippage threshold the driving torque of the internal combustion engine is reduced by a wheel slippage signal indicating the magnitude of the wheel slippage of a driven wheel.
In order to be able to transmit a certain lateral guide force between the driven wheels of a motor vehicle and the road surface, a noticeably greater wheel slippage is permitted in case of a high tire-road interface friction coefficient than in case of a low friction coefficient. For assuring a sufficiently large lateral guide force between the driven wheels of a motor vehicle and the road surface, also in case of a low friction coefficient, the slippage threshold value must therefore be designed for the low friction coefficient. This entails the disadvantage that in case of high friction coefficient, with which a greater wheel slippage would be permissive, potential acceleration capabilities of the motor vehicle cannot be utilized.
A system for the propulsion regulation in motor vehicles is already known from the DE-OS 35 45 652 in which one of three different slippage thresholds can be effectively engaged in dependence of a signal reproducing the longitudinal acceleration of the motor vehicle. As only three different slippage thresholds are provided with this known system, a matching of the slippage threshold to the different friction coefficients is possible only inadequately. As additionally the effective engagement of one of the three different slippage thresholds takes place in the known system in dependence on a signal indicating the longitudinal acceleration of the motor vehicle, the desired matching of the slippage threshold to the friction coefficient effective between the driven wheels of the motor vehicle and the road surface is also attained only inadequately even without this reason. This can be traced back to the fact that the measured longitudinal acceleration of the motor vehicle is only in a very rough relationship to the friction coefficient effective between the driven wheels of the motor vehicle and the road surface because the longitudinal acceleration of the motor vehicle, in addition to depending from the friction coefficient, also depends additionally from the yaw angle of the motor vehicle, on the one hand, and from the load of the motor vehicle, on the other. Finally, the known system is costly from a manufacturing point of view and therewith can be realized only in a relatively expensive manner because at least a separate longitudinal acceleration sensor is required. If therebeyond, for purposes of improving the regulating quality, the only very rough relationship of the measured longitudinal acceleration of the motor vehicle to the friction coefficient effective between the driven wheels of the motor vehicle and the road surface is to be improved, then additionally the steering angle has to be measured, respectively, determined with the assistance of a separate steering angle sensor and the load of the motor vehicle with the assistance of an additional load sensor. Even with the use of a longitudinal acceleration sensor, of a steering angle sensor and of a load sensor, an auxiliary signal determined with the assistance of these sensors produces changes of the friction coefficient between the driven wheels of the motor vehicle and the road surface only with delay because in case of an abrupt change of the friction coefficient between the driven wheels of the motor vehicle and the road surface, the longitudinal acceleration of the motor vehicle changes only with a delay in time by reason of the mass inertia of the motor vehicle.
It is therefore the object of the present invention to provide a regulating system of the aforementioned type by means of which an optimum as regards driving stability and propulsion of the motor vehicle is attained with minimum expenditure.
The underlying problems are solved according to the present invention in that the friction coefficient effective in each case in the wheel circumferential direction is determined as measure for the road condition and in that the threshold slippage determinative for the driven wheels is changed in dependence on the friction coefficient determined in each case and effective in the wheel circumferential direction.
As the friction coefficient effective in the wheel circumferential direction is in a very exact relationship to the friction coefficient altogether effective between the driven wheels of the motor vehicle and the road surface, a very exact and dynamically delay-free matching of the slippage threshold to the friction coefficient altogether effective between the driven wheels of the motor vehicle and the road surface is achieved by the change of the slippage threshold determinative for the driven wheels as a function of the respectively determined friction coefficient effective in the wheel circumferential direction. As a result of this matching of the slippage threshold to the friction coefficient effective at a given time between the driven wheels of the motor vehicle and the road surface, on the one hand, a sufficiently large lateral guidance force of the driven wheels of the motor vehicle is achieved in case of a low friction coefficient by reason of a correspondingly small slippage threshold and, on the other, in case of a high friction coefficient effective between the driven wheels of the motor vehicle and the road surface, an excellent propulsion or forward drive of the motor vehicle while maintaining at the same time a sufficiently large lateral guidance force of the driven wheels is achieved by reason of the then-correspondingly high slippage threshold. An optimum as regards driving stability and propulsion is achieved in this manner by the regulating system in accordance with the present invention with the most different friction coefficients.
For purposes of fine matching of the respective slippage threshold to the friction coefficient effective between the driven wheels of the motor vehicle and the road surface, the slippage threshold, according to another feature of the present invention, is continuously changed in dependence on the respectively determined friction coefficient effective in the wheel circumferential direction.
Preferably the slippage threshold is thereby changed proportionally to the respectively determined friction coefficient effective in the wheel circumferential direction.
According to another feature of the present invention, the velocity of the driving torque reduction taking place in case of a regulating interaction, respectively, of the subsequent driving torque increase is changed in dependence on the respectively determined friction coefficient effective in the wheel circumferential direction. It is achieved thereby that with a small friction coefficient effective between the driven wheels of the motor vehicle and the road surface a slow driving torque reduction, respectively, a subsequent slow driving torque increase takes place whereby the driven wheels of the motor vehicle are brought up to their slippage limit with slight torque excess. In case of a large friction coefficient effective between the driven wheels of the motor vehicle and the road surface, the velocity of the driving torque change can be considerably greater because, in this case, the excess driving torque is not excessive also with a very powerful internal combustion engine of the motor vehicle. The change of the velocity of the driving torque change in dependence on the respectively effective friction coefficient entails advantages as regards an optimum driving stability with best propulsion, especially with a non-homogeneous road surface.
According to a still further feature of the present invention, the velocity of the driving torque reduction, respectively, of the driving torque increase taking place in case of a regulating interaction is changed continuously in dependence on the respectively determined friction coefficient effective in the wheel circumferential direction. As a result of this continuous change, a very good matching of the respective driving torque change velocity to the respective friction coefficient effective between the driven wheels of the motor vehicle and the road surface is achieved.
Preferably, the velocity of the driving torque reduction, respectively, increase taking place in case of a regulating interaction is changed proportionally to the respectively determined friction coefficient effective in the wheel circumferential direction.
According to a further feature of the present invention, the respective friction load, i.e., the wheel circumferential force in relation to the respective wheel load and transmitted by a driven wheel onto the road surface is determined as function of the detected wheel slippage, i.e., the respective friction load-wheel slippage-characteristic curve is determined and the maximum of this friction load-wheel slippage-characteristic curve is determined at least approximately for determining the friction coefficient effective in each case in the wheel circumferential direction. In this manner, the respective friction coefficient effective in the wheel circumferential direction can be determined very accurately.
For determining the friction coefficient of the road side with the poorer road condition and therewith for taking into consideration the traction behavior of the more critical driven wheel, the friction load-wheel slippage-characteristic curve is determined in each case for the driven wheel at which the largest wheel slippage occurs.
Preferably, the at least approximate determination of the maximum of the respective friction load-wheel slippage-characteristic curve takes place by determining a characteristic drop of the characteristic curve slope. In this manner, the maximum of the respective friction load-wheel slippage-characteristic curve, i.e., the friction coefficient effective in the wheel circumferential direction, can be determined very rapidly so that the desired friction coefficient is available in each case without delay. This contributes considerably to a dynamically rapid and therewith very accurate regulation of the driving torque of the internal combustion engine of a motor vehicle.
According to a further advantageous construction of the present invention, the wheel circumferential force required in each case for the determination of the respective friction load is calculated on the basis of the driving torque of the internal combustion engine, the gear ratio of the transmission, the number of the driven wheels and the radius thereof.
The driving torque of the internal combustion engine of the motor vehicle is thereby determined according to a still further feature of the present invention in a simple manner on the basis of the position of the power-adjusting member, for example, of the throttle valve and of the rotational speed of the internal combustion engine.
Reference is thereby had advantageously to a stored family of characteristic curves reproducing the driving torque of the internal combustion engine in dependence on the position of the power output member and of the rotational speed of the internal combustion engine.
The wheel slippage necessary for determining the friction coefficient effective in the wheel circumferential direction, which is necessary in addition to the friction load, is detected according to another advantageous feature of the present invention by a comparison of the rotational speed of the driven wheel with the rotational speed of the non-driven wheel of a vehicle side.
The exact determination of the wheel slippage thereby takes place by subtraction of the rotational speed of the non-driven wheel from the rotational speed of the driven wheel and by subsequent division of the subtraction result by the rotational speed of the non-driven wheel.
In the alternative, the wheel slippage can be determined by subtraction of the motor vehicle velocity from the wheel circumferential velocity of a driven wheel and by subsequent division of the subtraction result by the vehicle velocity.